1. Field of the Invention
The present invention relates to an optical glass composition showing a low light scattering which is useful for optical glass components such as optical fibers for transmission, fiber lasers, and fiber amplifiers as well as to a method for preparing glasses useful for preparing glasses having such composition.
Further, the present invention relates to a low light scattering optical fiber which is useful for optical glass components such as optical fibers for transmission, fiber lasers, and fiber amplifiers as well as to a method for fabricating such an optical fiber.
2. Description of Related Art
Optical fibers made of silica glass have been put into practical use in optical communication. Loss margins of optical fibers are dominated by factors including intrinsic properties of glass, i.e., absorption due to molecular oscillation in infrared light region and scattering in visible light region (Rayleigh scattering). In order to obtain loss characteristics lower than that of current quartz optical fibers, there have been made extensive investigations with various glass materials. Generally, a lower loss would be obtained by the use of materials having lower scattering and lower absorption.
Fluoride glasses have been studied widely since they show excellent transmittance to infrared lights. However, upon preparation, they tend to precipitate microcrystals, which serve as centers of light scattering so that they have a high loss (J. B. MacChesney and D. J. DiGiovanni, J. Am Ceram. Soc., 1990, vol. 73, 3537-3556). This tendency of crystallization, which inherent to fluoride glasses, is difficult to avoid technically and, hence, there have been obtained no fluoride glass fiber that has loss characteristics superior to that of silica glass fiber.
On the other hand, some studies have been made with silicate glasses in order to attain low scattering. Silicate multicomponent oxide glasses are superior to fluoride glasses in that the former exhibits less crystal precipitation than the latter. However, the infrared transmission of the former is not so high as that of the latter so that it would be necessary to choose a glass composition which has low scattering in the ultraviolet-visible light region before a low loss optical fiber can be realized.
Of alkali-alkaline earth silicate glasses, there have been obtained glass compositions having lower Rayleigh scattering (J. Schroeder; "Light Scattering of Glass" in M. Tomozawa and R. H. Doremus, "Treatise on materials science and technology", Academic Press, New York, 1977, Vol. 12, p. 199 [Na.sub.2 O--MgO--SiO.sub.2 Glass], and G. A. C. M. Spierings and T. P. M. Meeuwsen; J. Non-Crystl. Solids, 1984, Vol. 66, p. 494 [K.sub.2 O-MgO-SiO.sub.2 Class]).
In their earlier studies (S. Todoroki and S. Sakaguchi, J. Am. Cer. Soc., 78[9], 2566-68 (1995) and unpublished work), the present inventors have found that oxide compositions, or more specifically, silicate glass compositions composed of silicon dioxide (SiO.sub.2), a divalent metal oxide (M.sup.II O, where M.sup.II represents a divalent cation), and a monovalent metal compound (M.sub.2.sup.I O, where M.sup.I is a monovalent cation) which contain 50 to 80 mol % of SiO.sub.2, 0 to 20 mol % of M.sup.II O, and 15 to 40 mol % of M.sub.2.sup.I O have lower light scattering than pure SiO.sub.2 glass and can vitrify stably than pure SiO.sub.2 glass. M.sup.II may be at least one element selected from the group consisting of Mg, Ca, Sr, Ba, and Zn. On the other hand, M.sup.I may be at least one element selected from the group consisting of Li, Na, and K.
The components of the above-described silicate glasses are limited since crystals tend to precipitate when the SiO.sub.2 content is less than 50 mol %, or the M.sub.2.sup.I O content is more than 40 mol %, while light scattering increases when the SiO.sub.2 content is more than 80 mol %, the M.sup.II O content is more than 20 mol %, or the M.sub.2.sup.I O content is less than 15 mol %.
However, there has been no attempt to fabricate optical fibers from the above-described silicate glass compositions. In other words, there has been known no glass that can provide glass compositions which have slightly different refractive indices for use as core and clad, which show low scattering, and which are thermally stable enough to fabricate optical fibers.
In the case where optical fibers are fabricated from the above-mentioned glasses found in the earlier studies by the present inventors, in order to form a waveguide structure, there are needed two kinds of glasses having similar thermal properties but different refractive indices, one for the core and another for the clad. In order to obtain equivalent thermal properties, it would be better to use glasses as close in composition as possible. For this purpose, generally, there are two approaches:
(1) To dope a core glass with a component which increases its refractive index; and
(2) To dope a clad glass with a component which decreases its refractive index.
In the case of (1) above, the increase in refractive index results in an increased Rayleigh scattering. That is, the Rayleigh scattering inherent to glasses increases in proportion to the 8th power of n, refractive index, as expressed by the following equation (D. A. Pinnow et al., Appl. Phys. Lett., Vol. 22, p. 527 (1973)): ##EQU1## wherein n is the refractive index;
k is Boltzmann constant;
p is the photoelastic constant;
K.sub.T (T.sub.F) is the static isothermal compression; and
T.sub.F is the fictive temperature, which is equivalent to the glass transition temperature (Tg).
In fact, an increased scattering of a glass doped with Pb has been reported (G. A. C. M. Spierings and T. P. M. Meeuwsen; J. Non-Cryst. Solids, 1984, Vol. 66, p. 464).
In the case of (2) above, it has been known that doping of a silica glass with boron (B) or fluorine (F) decreases Tg and n (T. Ixawa and S. Sudo, "Optical fibers: materials and fabrication", KTK Scientific Publishers, p. 27 (1987). Accordingly, it is considered effective to dope a glass with boron (B) or fluorine (F) in order to decrease its refractive index. However, boron increases infrared absorption of a glass when doped therein and, hence, it is ineffective for loss reduction. Thus, introduction or doping of fluorine would be considered hopeful.
However, as far as the present inventors know, there has been no report on fluorine-doped silicate glass having low Rayleigh scattering although many examples of fluorine-doped silicate glass compositions were known.
For example, some optical glass compositions contain fluorine (Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, Vol. A18. (1991) p. 201). In spite of many investigations on Rayleigh scattering of optical glasses as described in, for example, J. Schroeder et al., J. Non-Cryst. Solids, Vol. 13, pp. 313-320, Y. Moriya et al., Yogyo-Kyokai-Shi, Vol. 22, p. 527 (1973), there has been known no glass that has a loss lower than the loss of silica glass currently used for optical fibers as low as 1.1 dB/km at 0.8 .mu.m; D. A. Pinnow et al.; Appl. Phys. Lett., Vol. 22, p. 527 (1973).
The reason for this would be that introduction of fluorine in silicate glasses induces generation of a heterogeneous structure such as phase separation or precipitation of crystals. In fact, in the glass industries, fluorides are generally used in order to obtain opal glasses or crystallized glasses (H. Scholze, "Glass--Nature, Structure and Properties", Springer-Verlag, New York, 1991). Degree of phase separation or crystallization relates to glass composition closely. However, there has been no report that describes specific silicate oxyfluoride glass composition which forms stable glass and has a low light scattering.
As described above, oxyfluoride silicate glasses have low refractive indices and low glass transition temperatures and, hence, are expected to be materials which show low light scattering. However, it is known that fluorine in silicate glass causes phase separation and crystallization, resulting in light scattering. Therefore, relation between light scattering intensity and composition of glass remains to be clarified.
In other words, there is known no clad glass that has a low refractive index, shows a low scattering and is thermally stable, the clad glass being necessary to fabricate an optical fiber with a silicate glass core having a low Rayleigh scattering.
Optical glass components, whose representative example is an optical fiber, must be highly pure and homogeneous so that attenuation of light which transmits therein can be minimized. In a conventional method for the manufacture of such glasses, starting materials are introduced in a crucible and heated until they are molten. Therefore, it is inevitable that impurities from the crucible contaminate the glass.
As a method for solving the problem, there has been known a vapor phase synthesis represented by vapor phase axial deposition (VAD) method. This method, which does not involve a liquid state, needs no vessel so that contamination of impurities from a vessel can be prevented and enables one to carry out high purity synthesis. Further, since a heated liquid generates a convection current, the glass which has been once molten tends to form striae, i.e., portions which have uneven refractive indices, while the vapor phase deposition methods can produce homogeneous glass without striae.
However, the vapor phase deposition methods have a limitation in that metals having relatively low vapor pressure such as alkali metals, alkaline earth metals, and rare earth metals cannot be incorporated. And, it is often needed to introduce such low vapor pressure metals into glass compositions in order to meet various optical properties required for optical components.
As a method for introducing such metal elements, there has been used in combination a solution doping method (J. B. MacChesney and D. J. DiGiovanni, "Materials Development of Optical Fiber", J. Am. Ceram. Soc., 73, 12 (1990) 3537-3556).
In the solution doping method, a porous glass matrix is used which is prepared by depositing fine particles of a glass formed by flame hydrolysis of a metal compound in vapor phase, for example, a halide such as SiCl.sub.4, together with H.sub.2 and O.sub.2. The matrix is dipped in a solution of a metal compound, taken out from the solution, dried to remove the solvent, and sintered at high temperatures to form a transparent glass. The use of the solution doping method in combination enables one to introduce low vapor pressure metals without deteriorating the advantage of the vapor phase synthesis, i.e., less contamination of impurities.
However, in the above-described solution doping method, evaporation (gasification from a liquid phase) of the solvent near the surface of the porous glass matrix occurs so that the solvent which is mobile at the time of evaporation transports the solute to near the surface of the matrix and deposition of the dopant on the surface occurs inevitably.
As a result, if the bulk density of the porous glass matrix is uniform, the solvent and solute distribute uniformly before drying as shown in FIG. 1 in broken lines Ca and Cb, respectively, and according as the drying proceeds, the solute distributes in a higher concentration in a position nearer the surface as shown in FIG. 2 in broken line Cb.
As described above, the solution doping method has a disadvantage that upon evaporation the solvent transports the solute, resulting in excessive accumulation of the metal compound near the surface of the matrix.
In order to solve this problem, there has been known a method in which the dopant which was localized near the surface of the porous glass matrix is washed away (Japanese Patent Application Laying-open No. 300224/1992).
However, in this method, only a portion of the solute which is contained in the solution used for the solution doping is doped into the porous glass matrix and, hence, the amount of the metal doped is limited. Accordingly, the glass compositions which can be prepared by this method are limited.
As described above, in the preparation of glass by a gas phase synthesis along with a solution doping method, glasses that can be synthesized by the method have only limited compositions because of localized deposition of the metal compound used for doping near the surface of the porous glass matrix.